Carbon nanoparticle-containing hydrophilic nanofluid with enhanced thermal conductivity

ABSTRACT

The present invention relates to a nanofluid that contains carbon nanoparticles, metal oxide nanoparticles and a surfactant in a thermal transfer fluid. The present invention also relates to processes for producing such a nanofluid with enhanced thermal conductive properties.

TECHNICAL FIELD

The present invention relates to a nanofluid that contains carbonnanoparticles, metal oxide nanoparticles and a surfactant in a thermaltransfer fluid. The present invention also relates to processes forproducing such a nanofluid with enhanced thermal conductive properties.

BACKGROUND OF THE INVENTION

Conventional heat transfer fluids such as water, mineral oil, andethylene glycol play an important role in many industries includingpower generation, chemical production, air conditioning, transportation,and microelectronics. However, their inherently low thermalconductivities have hampered the development of energy-efficient heattransfer fluids that are required in a plethora of heat transferapplications. It has been demonstrated recently that the heat transferproperties of these conventional fluids can be significantly enhanced bydispersing nanometer-sized solid particles and fibers (i.e.nanoparticles) in fluids (Eastman, et al., Appl. Phys. Lett. (2001),78(6):718-720; Choi, et al., Appl. Phys. Lett. (2001),79(14):2252-2254). This new type of heat transfer suspension is referredto herein as a nanofluid. In particular, carbon nanotube-containingnanofluids provide several advantages over conventional fluids,including thermal conductivities far above those of traditionalsolid/liquid suspensions, a nonlinear relationship between thermalconductivity and concentration, strongly temperature-dependent thermalconductivity, and a significant increase in critical heat flux. Each ofthese features is highly desirable for thermal systems and together,they make nanofluids good candidates for the next generation of heattransfer fluids.

The observed substantial increases in the thermal conductivities ofnanofluids can have broad industrial applications and can alsopotentially generate numerous economical and environmental benefits.Enhancement in the heat transfer ability could translate into highenergy efficiency, better performance, and low operating costs. The needfor maintenance and repair can also be minimized by developing ananofluid with a better wear and load-carrying capacity. Consequently,classical heat dissipating systems widely used today can become smallerand lighter, thus resulting in better fuel efficiency, less emission,and a cleaner environment.

Nanoparticles of various materials have been used individually to makeheat transfer fluids, including copper, aluminum, copper oxide, alumina,titania, and carbon nanotubes (Keblinski, et al, Material Today, (2005),36-44). Of these nanoparticles, carbon nanotubes show the greatestpromise due to their excellent chemical stability and extraordinarythermal conductivity. Carbon nanotubes are macromolecules having theshape of a long thin cylinder and thus have a high aspect ratio. Thereare two main types of carbon nanotubes: single-walled nanotubes (“SWNT”)and multi-walled nanotubes (“MWNT”). The structure of a single-walledcarbon nanotube can be described as a single graphene sheet rolled intoa seamless cylinder with ends that are either open, or capped by eitherhalf fullerenes or more complex structures such as pentagons.Multi-walled carbon nanotubes contains two or more nanotubes that areconcentrically nested, like rings of a tree trunk, with a typicaldistance of approximately 0.34 nm between layers.

Carbon nanotubes are one of the most thermally conductive materialsknown today. Basic research over the past decade has shown that carbonnanotubes have a thermal conductivity an order of magnitude higher thancopper (3,000 watts per meter Kevin (W/mK) for multi-walled carbonnanotubes and 6,000 W/mK for single-walled carbon nanotubes). Therefore,the thermal conductivities of nanofluids containing carbon nanutubes aresignificantly enhanced when compared with conventional fluids. A 150%increase in conductivity of oil that contains about 1% by volume ofmulti-walled carbon nanotubes has been reported recently (Choi, et al.,App. Phys. Lett., (2001), 79(14):2252-2254).

Several additional studies of carbon nanotube suspensions in variousheat transfer fluids have also been reported. However, only moderateenhancements in thermal conductivity have been observed. Xie, et al.,reported that a carbon nanotube suspension in an aqueous solution oforganic liquids results in only 10-20% increases in thermal conductivityat 1% by volume of carbon nanotubes (Xie, et al., J. Appl. Phys.,(2003), 94(8):4967-4971). Similarly, Wen and Ding found an about 25%enhancement in the conductivity at about 0.8% by volume of carbonnanotubes in water (Wen and Ding, J. Thermophys. Heat Trans., (2004),18:481-483). Even at these levels, carbon nanotubes still hold greatpromise as being the next generation of efficient thermal transferfluids.

Despite the extraordinarily promising thermal properties exhibited bycarbon nanotube suspensions, it remains to be a serious technicalchallenge to effectively and efficiently disperse carbon nanotubes intoaqueous or organic media to produce a nanoparticle suspension with asustainable stability and having consistent thermal properties. Due tothe hydrophobic nature of graphitic structure, unmodified carbonnanotubes are not soluble in any known solvent. They also have a veryhigh tendency to form aggregates and extended structures of linkednanoparticles, thus leading to phase separation, poor dispersion withina matrix, and poor adhesion to the host. However, stability of thenanoparticle suspension is essential for practical industrialapplications. Otherwise, the thermal properties of a nanofluid, such asthermal conductivity, will constantly change as the solid nanoparticlesgradually separate from the fluid. Unfortunately, these early studies oncarbon nanotube-containing nanofluids have primarily focused on theenhancement of thermal conductivity, and very little experimental datais available regarding the stability of these nanoparticle suspensions.

Accordingly, there is a need for the development of a stablenanoparticle-containing fluid and methods for efficiently dispersingcarbon nanoparticles into a desired heat transfer fluid to produce ananofluid with a sustainable stability and consistent thermalproperties. Hence, the present invention provides a nanofluid, whichcomprises a conventional heat transfer fluid, carbon nanoparticles,metal oxide nanoparticles and a surfactant. The metal oxidenanoparticles in combination with the surfactant are used to facilitatethe dispersion of the carbon nanoparticles and to increase the stabilityof the nanofluid. The present invention also provides methods forpreparing such carbon nanoparticle-containing fluid with enhancedthermal conductive properties.

SUMMARY OF THE INVENTION

The present invention provides a nanofluid with enhanced thermalconductive properties, which comprises a conventional thermal transferfluid, and carbon and non-carbon nanoparticles.

In accordance with an embodiment of the present invention, the nanofluidcomprises a hydrophilic thermal transfer fluid, carbon nanoparticles,metal oxide nanoparticles and a surfactant. The hydrophilic nanofluidmay further comprise other chemical additives to enhance thecharacteristics of the nanofluid.

Accordingly, in one embodiment, the nanofluid of the present inventionis a composition comprising carbon nanoparticles, metal oxidenanoparticles having a pHpzc, and at least one surfactant having a netnegative charge in a hydrophilic thermal transfer fluid, wherein thecomposition has a pH below the pHpzc of the metal oxide nanoparticles.

The nanoparticles of the present invention may include, for example,diamond nanoparticles, graphite nanoparticles, fullerenes, carbonnanotubes, and carbon fibers. The carbon nanotubes may be single walled(SWNTs) or multi-walled (MWNTs.)

The hydrophilic thermal transfer fluid may be, for example, water, alkylalcohols, alkylene glycols, and combinations thereof. Further thealkylene glycol can be ethylene glycol or diethylene glycol.

Exemplary metal oxides are MgO, CuO and Al₂O₃.

Exemplary surfactants are sodium dodecylbenzenesulfonate (SDBS) andsodium dodecyl sulfate (SDS).

In a specific example, the nanofluids comprise SWNTs, along with metaloxide (MgO, CuO or Al₂O₃) nanoparticles, and SDS or SDBS, at a pH ofbetween 7 and 9.

The present invention also includes a method for making a nanofluidcomprising the steps of: mixing together carbon nanoparticles, metaloxide nanoparticles having a pHpzc, and at least one surfactant having anet negative charge into a hydrophilic thermal transfer fluid; andadjusting the pH below the pHpzc of the metal oxide nanoparticles.

Other aspects of the present invention are found throughout thespecification.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to nanofluids that comprise carbonnanoparticles, metal oxide nanoparticles and a surfactant in a thermaltransfer fluid. Additionally, the nanofluid may further comprisechemical additives to provide other desired chemical and physicalcharacteristics, such as anti-wear, corrosion protection and thermaloxidative properties.

While not wishing to be bound by any particular scientific theory, it isbelieved that there is a charge attraction between the nonpolar regionof the surfactant molecules and the carbon nanoparticles. Thisinteraction forms a shell around the carbon nanoparticles, with thecharged head region of the surfactant molecules oriented towards theoutside. This facilitates the dispersion of the carbon nanoparticles inthe fluid thus preventing precipation from the fluid, which in turnenhances thermal conductivity. To further enhance thermal conductivity,metal oxide nanoparticles are also added to the thermal transfer fluid.These positively charged metal oxide nanoparticles repel one another andfurther enhance stability and thermal conductivity of the nanofluid.

As used in this disclosure, the singular forms “a”, “an”, and “the” mayrefer to plural articles unless specifically stated otherwise. Tofacilitate understanding of the invention set forth in the disclosurethat follows, a number of terms are defined below.

Definitions:

The term “aspect ratio” refers to a ratio of the length over thediameter of a particle.

The term “carbon nanoparticle” refers to a nanoparticle which containsprimarily carbon, including diamond, graphite, fullerenes, carbonnanotubes, carbon fibers, and combinations thereof.

The term “SWNT” refers to a single-walled carbon nanotube.

The term “D-SWNT” refers to a double-walled carbon nanotube.

The term “MWNT” refers to a multi-walled carbon nanotube that containstwo or more walls. Thus, D-SWNT is included in the definition of MWNT.

The term “F-SWNT” refers to a fluorinated SWNT.

The term “carbon nanotube” refers to SWNT, MWNT, and structuralvariations and modification thereof, including configurations,structural defects and variations, tube arrangements, chemicalmodification and functionalization, and encapsulation.

The term “nanoparticle” refers to a particle having at least onedimension that is no greater than 500 nm, and sometimes no greater than100 nm, and includes, for example, “nanospheres,” “nanorods,”“nanocups,” “nanowires,” “nanoclusters,” “nanolayers,” “nanotubes,”“nanocrystals,” “nanobeads,” “nanobelts,” and “nanodisks.”

The term “nanoscale” refers to a dimension that is no greater than 500nm, and sometimes no greater than 100 nm. The terms “nanoscale particle”and “nanoparticle” are used interchangeably in the present invention.

The term “pH point of zero charge”, or “pHpzc” refers to the pH value ofa metal oxide nanoparticle-containing fluid at which the metal oxidenanoparticle exhibits a neutral surface charge. The pHpzc can bemeasured by any known method, such as the one described by Bourikas, etal., Environ. Sci. Technol., 39(11): 4100-4108 (2005).

The term “surfactant” refers to a molecule having surface activity,including wetting agents, dispersants, emulsifiers, detergents, andfoaming agents, etc.

Carbon Nanoparticles:

Carbon nanoparticles have a high heat transfer coefficient and highthermal conductivity, which often exceed these of the best metallicmaterial. The carbon nanoparticles for use in the present inventioncomprise elemental carbon. Many forms of carbon nanoparticles aresuitable for use in the present invention, including carbon nanotubes,diamonds, fullerenes, graphite, carbon fibers, and combinations thereof.

Carbon nanotubes (“CNT”) are macromolecules in the shape of a long, thincylinder, often with a diameter of a few nanometers. The basicstructural element in a carbon nanotube is a hexagon, which is the sameas that found in graphite. Based on the orientation of the tube axiswith respect to the hexagonal lattice, a carbon nanotube potentially hasthree different configurations: armchair, zigzag, and chiral (also knownas spiral). In armchair configuration, the tube axis is perpendicular totwo of six carbon-carbon bonds of the hexagonal lattice. In zigzagconfiguration, the tube axis is parallel to two of six carbon-carbonbonds of the hexagonal lattice. Both these two configurations areachiral. In chiral configuration, the tube axis forms an angle otherthan 90 or 180 degrees with any of six carbon-carbon bonds of thehexagonal lattice. Nanotubes with these three configurations oftenexhibit different physical and chemical properties. For example, anarmchair nanotube is always metallic, whereas a zigzag nanotube may bemetallic or semiconductive, depending on the diameter of the nanotube.All three types of different configurations of nanotubes are expected tobe very good thermal conductors along the tube axis, exhibiting aproperty known as “ballistic conduction,” but are also good insulatorslaterally to the tube axis.

In addition to the common hexagonal structure, the cylinder of a carbonnanotube may also contain other sized rings, such as pentagons andheptagons. Replacement of some regular hexagons with pentagons and/orheptagons may cause cylinders to bend, twist, or change diameter, andthus lead to some interesting structures such as “Y-,” “T-,” and“X-junctions,” and different physical and chemical properties. Thesestructural variations and configurations can be found in both SWNT andMWNT. The present invention is not limited to any particularconfiguration and structural variation. The carbon nanotubes used in thepresent invention may be in the configuration of armchair, zigzag,chiral, or combinations thereof. The carbon nanotubes may also containstructural elements other than hexagon, such as pentagon, heptagon,octagon, or combinations thereof.

Another structural variation, which is specific for MWNT molecules, isthe arrangement of multiple tubes in a MWNT molecule. A perfect MWNT islike a stack of graphene sheets rolled up into concentric cylinders,with each wall parallel to the central axis. However, the tubes may alsobe arranged so that an angle between the graphite basal planes and thetube axis is formed. Such MWNTs are known as a “stacked cone”,“Chevron,” “bamboo,” “ice cream cone,” or “piled cone structure.” Astacked cone MWNT may reach a diameter of about 100 nm. In spite ofthese structural variations, all MWNTs are suitable for the presentinvention, as long as they have excellent thermal conductive properties.

Carbon nanotubes used in the present invention may also encapsulateother elements and/or molecules within their enclosed tubularstructures. Such elements include Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y,Zr, Mo, Ta, Au, Th, La, Ce, Pr, Nb, Gd, Th, Dy, Ho, Er, Tm, Yb, Lu, Mo,Pd, Sn, and W. Such molecules include alloys of these elements such asalloys of Cobalt with S, Br, Pb, Pt, Y, Cu, B, and Mg, and compoundssuch as the carbides (i.e. TiC, MoC, etc.) The present of theseelements, alloys and compounds within the core structure of fullerenesand nanotubes may enhance the thermal conductivity of thesenanoparticles, which then translates to a nanofluid with a higherthermal conductivity when these nanoparticles are suspended in a heattransfer fluid.

Carbon nanoparticles used in the present invention may also bechemically modified or functionalized. Covalent functionalization ofcarbon nanoparticles, especially carbon nanotubes and fullerenes, hascommonly been accomplished by three different approaches, namely,thermally activated chemistry, electrochemical modification, andphotochemical functionalization. The most common method of thermallyactivated chemical functionalization is an addition reaction on thesidewalls. For example, the extensive treatment of a nanotube withconcentrated nitric and sulfuric acids leads to the oxidative opening ofthe tube caps as well as the formation of holes in the sidewalls. Thisproduces a nanotube decorated with carboxyl groups, which can be furthermodified through the creation of amide and ester bonds to generate avast variety of functional groups. The nanotube molecule may also bemodified through addition reactions with various chemical reagents, suchhalogens and ozone. Unlike thermally controlled modification,electrochemical modification of nanotubes may be carried out in a moreselective and controlled manner. Interestingly, a SWNT can beselectively modified or functionalized either on the cylinder sidewallor the optional end caps. These two structurally distinct moieties oftendisplay different chemical and physical characteristics. The functionalgroups on functionalized carbon nanoparticles may be attached directlyto the carbon atoms of a carbon nanoparticle or via chemical linkers,such as alkylene or arylene groups. To increase hydrophilicity, carbonnanoparticles may be functionalized with one or more hydrophilicfunctional groups, such as sulfonate, carboxyl, hydroxyl, amino, amide,urea, carbamate, urethane, phosphate, and/or functionalized alkyl andaryl groups (e.g., sulfonated p-aminophenyl group). To increasehydrophobicity, carbon nanoparticles may be functionalized with one ormore hydrophobic alkyl or aryl groups. The functionalized carbonparticle may have no less than about 2, no less than about 5, no lessthan about 10, no less than about 20, or no less than about 50functional groups on average.

Carbon nanotubes are commercially available from a variety of commercialsources. SWNTs may be obtained from Carbolex (Broomall, Pa.), MERCorporation (Tucson, Ariz.), and Carbon Nanotechnologies Incorporation(“CNI”, Houston, Tex.). MWNTs may be obtained from MER Corporation(Tucson, Ariz.) and Helix material solution (Richardson, Tex.). However,the present invention is not limited by the source of carbon nanotubes.In addition, many publications are available with sufficient informationto allow one to manufacture nanotubes with desired structures andproperties. Currently, the most common techniques are arc discharge,laser ablation, chemical vapor deposition, and flame synthesis. Ingeneral, the chemical vapor deposition has shown the most promise inbeing able to produce larger quantities of nanotubes at lower cost. Thisis usually done by reacting a carbon-containing gas, such as acetylene,ethylene, ethanol, etc., with a metal catalyst particle, such as cobalt,nickel, or ion, at temperatures above 600° C. The present invention isalso not limited by the manufacturing method of the carbon nanotubes.

The selection of a particular carbon nanoparticle depends on a number offactors. The most important one is that the nanoparticle has to becompatible with an already existing base fluid (i.e., a thermal transferfluid) discussed elsewhere herein. Other factors include heat transferproperties, cost effectiveness, solubility, dispersion and settlingcharacteristics. The carbon nanoparticule-containing nanofluid maycontain predominantly SWNTs, MWNTs, or mixtures thereof. The carbonnanoparticles in the nanofluid may also be chemically functionalized.The functionalized carbon nanoparticles may be soluble in a hydrophilicthermal transfer fluid, and are thus suitable for preparing ahydrophilic nanofluid. Alternatively, they may be soluble in ahydrophobic thermal transfer fluid, and are thus suitable for preparinga hydrophobic nanofluid.

In general, carbon nanotubes are characterized by physical and chemicalproperties, such as a carbon content, diameter, length, aspect ratio,and thermal conductivity. The carbon nanotubes suitable for use in thepresent invention may have a carbon content of no less than about 60%,no less than about 80%, no less than about 90%, no less than about 95%,no less than about 98%, or no less than about 99%. The carbon nanotubesmay have a diameter of from about 0.2 to about 100 nm, from about 0.4 toabout 80 nm, from about 0.5 to about 60 nm, or from about 0.5 to about50 nm. The carbon nanotubes may have a length of no greater than about200 micrometers, no greater than 100 micrometers, no greater than about50 micrometers, or no greater than 20 micrometers. Furthermore, thecarbon nanotubes may have an aspect ratio of no greater than 1,000,000,no greater than 100,000, no greater than 10,000, no greater than 1,000,no greater than about 500, no greater than about 200, or no greater thanabout 100. Alternatively, the carbon nanotubes may have a thermalconductivity of no less than 10 W/m·K, no less than 100 W/m·K, no lessthan 500 W/m·K, or no less than 1,000 W/m·K.

For a functionalized carbon nanotube, it may be further characterized byits solubility in a thermal transfer fluid. The carbon nanotubes used inthe present invention may have a solubility of no less than 0.01 g/L, noless than 0.05 g/L, no less than 0.1 g/L, no less than 0.5 g/L, no lessthan 1 g/L, no less than 2 g/L, no less than 5 g/L, or no less than 10g/L in a desired thermal transfer fluid, either hydrophilic orhydrophobic.

The carbon particles suitable for use in the present invention may bediamond nanoparticles, graphite nanoparticles, fullerenes, or carbonfibers. Furthermore, the carbon nanoparticles may be a combination oftwo or more selected from diamond nanoparticles, graphite nanoparticles,fullerenes, carbon fibers, and carbon nanotubes. A combination may be amixture of two or more nanoparticles of the same type or of differenttypes. For examples, a combination of two nanoparticles can be a mixtureof SWNT and MWNT, a mixture of two SWNTs with different properties, amixture of two MWNT with different properties, a mixture of carbonnanotubes with graphite nanoparticles, a mixture of carbon nanotubeswith diamond particles, and a mixture of carbon nanotubes withfullerenes.

Metal Oxide Nanoparticles:

A metal oxide nanoparticle is a nanoscale particle that comprises one ormore metal oxides. Such metal oxides include, for example, those formedfrom metal and/or metalloid, either in elemental form and/or incompounds. Suitable metal/metalloid oxides include but are not limitedto Al₂O₃, CuO, MgO, SiO₂, GeO₂, B₂O₃, TeO₂, V₂O₅, BiO₂, Sb₂O₅, TiO₂,ZnO, FeO, Fe₂O₃, Fe₃O₄, and CrO₃. As used herein, the chemical formulafor a metal oxide nanoparticle refers to the chemical with that formulathat is a component, usually the principal component, of thenanoparticle. The chemical may be a major or minor component of thenanoparticle. As such, the nanoparticle may not have the same chemicalcomposition as the chemical formula. Furthermore, unless specified, thechemical formula of a nanoparticle represents any of the possiblecrystalline forms. For example, the chemical formula Al₂O₃ may representalpha-, beta-, or gamma-aluminum oxide, or combinations thereof.

Suitable metal oxide nanoparticles for use in the present invention mayhave, for example, a pHpzc of between 6 and 10, 7 and 10, 8 and 10, and9 and 10, etc. For example, the exemplary metal oxides, MgO, CuO andAl₂O₃ have a pHpzc between 9 and 10.

Thermal Transfer Fluid:

In the present invention, the major component of the nanofluid is ahydrophilic thermal transfer fluid. The hydrophilic thermal transferfluid of the present invention includes a hydrophilic liquid that ismiscible with water, including water, aliphatic alcohols, alkyleneglycols, di(alkylene)glycols, monoalkyl ethers of alkylene glycols ordi(alkylene)glycols, and various mixtures thereof. Suitable aliphaticalcohols contain no greater than 6 carbons and no greater than 4hydroxyls, such as methanol, ethanol, isopropanol, and glycerol.Suitable alkylene glycols contain no greater than 5 carbons, such asethylene glycol, propylene glycol, and 1,2-butylene glycol.Particularly, the hydrophilic thermal transfer fluid comprises ethyleneglycol, propylene glycol, and mixtures thereof. Ethylene glycol andpropylene glycol are excellent antifreeze agents and also markedlyreduce the freezing point of water. Suitable di(alkylene)glycols containno greater than 10 carbons, such as diethylene glycol, triethyleneglycol, tetraethylene glycol, and dipropylene glycol. Commercialantifreeze coolants often contain more than one glycol compounds. Forexample, PRESTONE™ antifreeze (Honeywell Consumer Products Group,Danbury, Conn.) contains 95 to 100% of ethylene glycol and no greaterthan 5% of diethylene glycol. The mixture as used herein refers to acombination of two or more hydrophilic liquids. As used herein, the term“alkylene glycol” refers to a molecule having glycol functional moietyin its structure in general, including alkylene glycol, alkyleneglycols, di(alkylene)glycols, tri(alkylene)glycols,tetra(alkylene)glycols, and their various derivatives, such as ethersand carboxylic esters.

Surfactant:

A variety of surfactants may alternatively be included in the presentinvention as a dispersant to facilitate uniform dispersion ofnanoparticles in a desired thermal transfer fluid, and to enhancestabilization of such a dispersion as well. Typically, the surfactantsused in the present invention contain an lipophilic nonpolar hydrocarbongroup and a polar functional hydrophilic group. The polar functionalgroup may be a carboxylate, ester, amine, amide, imide, hydroxyl, ether,nitrile, phosphate, sulfate, or sulfonate. The surfactants that areuseful in the present invention may be used alone or in combination.Accordingly, any combination of surfactants may include anionic,cationic, nonionic, zwitterionic, amphoteric and ampholytic surfactants,so long as there is a net positive charge in the head regions of thepopulation of surfactant molecules. In most instances, a singlenegatively charged surfactant is used in the preparation of thenanofluids of the present invention.

Accordingly, the surfactants for use in the present invention may beanionic, including, but not limited to, sulfonates such as alkylsulfonates, alkylbenzene sulfonates, alpha olefin sulfonates, paraffinsulfonates, and alkyl ester sulfonates; sulfates such as alkyl sulfates,alkyl alkoxy sulfates, and alkyl alkoxylated sulfates; phosphates suchas monoalkyl phosphates and dialkyl phosphates; phosphonates;carboxylates such as fatty acids, alkyl alkoxy carboxylates,sarcosinates, isethionates, and taurates. Specific examples ofcarboxylates are sodium cocoyl isethionate, sodium methyl oleoyltaurate, sodium laureth carboxylate, sodium trideceth carboxylate,sodium lauryl sarcosinate, lauroyl sarcosine, and cocoyl sarcosinate.Specific examples of sulfates include sodium dodecyl sulfate (SDS),sodium lauryl sulfate, sodium laureth sulfate, sodium trideceth sulfate,sodium tridecyl sulfate, sodium cocyl sulfate, and lauric monoglyceridesodium sulfate.

Suitable sulfonate surfactants include, but are not limited to, alkylsulfonates, aryl sulfonates, monoalkyl and dialkyl sulfosuccinates, andmonoalkyl and dialkyl sulfosuccinamates. Each alkyl group independentlycontains about two to twenty carbons and can also be ethoxylated with upto about 8 units, preferably up to about 6 units, on average, e.g., 2,3, or 4 units, of ethylene oxide, per each alkyl group. Illustrativeexamples of alky and aryl sulfonates are sodium tridecyl benzenesulfonate (STBS) and sodium dodecylbenzene sulfonate (SDBS).

Illustrative examples of sulfosuccinates include, but are not limitedto, dimethicone copolyol sulfosuccinate, diamyl sulfosuccinate, dicaprylsulfosuccinate, dicyclohexyl sulfosuccinate, diheptyl sulfosuccinate,dihexyl sulfosuccinate, diisobutyl sulfosuccinate, dioctylsulfosuccinate, C12-15 pareth sulfosuccinate, cetearyl sulfosuccinate,cocopolyglucose sulfosuccinate, cocoyl butyl gluceth-10 sulfosuccinate,deceth-5 sulfosuccinate, deceth-6 sulfosuccinate, dihydroxyethylsulfosuccinylundecylenate, hydrogenated cottonseed glyceridesulfosuccinate, isodecyl sulfosuccinate, isostearyl sulfosuccinate,laneth-5 sulfosuccinate, laureth sulfosuccinate, laureth-12sulfosuccinate, laureth-6 sulfosuccinate, laureth-9 sulfosuccinate,lauryl sulfosuccinate, nonoxynol-10 sulfosuccinate, oleth-3sulfosuccinate, oleyl sulfosuccinate, PEG-10 laurylcitratesulfosuccinate, sitosereth-14 sulfosuccinate, stearyl sulfosuccinate,tallow, tridecyl sulfosuccinate, ditridecyl sulfosuccinate, bisglycolricinosulfosuccinate, di(1,3-di-methylbutyl)sulfosuccinate, and siliconecopolyol sulfosuccinates. The structures of silicone copolyolsulfosuccinates are set forth in U.S. Pat. Nos. 4,717,498; and4,849,127.

Illustrative examples of sulfosuccinamates include, but are not limitedto, lauramido-MEA sulfosuccinate, oleamido PEG-2 sulfosuccinate,cocamido MIPA-sulfosuccinate, cocamido PEG-3 sulfosuccinate,isostearamido MEA-sulfosuccinate, isostearamido MIPA-sulfosuccinate,lauramido MEA-sulfosuccinate, lauramido PEG-2 sulfosuccinate, lauramidoPEG-5 sulfosuccinate, myristamido MEA-sulfosuccinate, oleamidoMEA-sulfosuccinate, oleamido PIPA-sulfosuccinate, oleamido PEG-2sulfosuccinate, palmitamido PEG-2 sulfosuccinate, palmitoleamido PEG-2sulfosuccinate, PEG-4 cocamido MIPA-sulfosuccinate, ricinoleamidoMEA-sulfosuccinate, stearamido MEA-sulfosuccinate, stearylsulfosuccinamate, tallamido MEA-sulfosuccinate, tallow sulfosuccinamate,tallowamido MEA-sulfosuccinate, undecylenamido MEA-sulfosuccinate,undecylenamido PEG-2 sulfosuccinate, wheat germamido MEA-sulfosuccinate,and wheat germamido PEG-2 sulfosuccinate.

Some examples of commercial sulfonates are AEROSOL® OT-S, AEROSOL®OT-MSO, AEROSOL® TR70% (Cytec Inc., West Paterson, N.J.), NaSul CA-HT3(King Industries, Norwalk, Conn.), and C500 (Crompton Co., West Hill,Ontario, Canada). AEROSOL® OT-S is sodium dioctyl sulfosuccinate inpetroleum distillate. AEROSOL® OT-MSO also contains sodium dioctylsulfosuccinate. AEROSOL® TR70% is sodium bistridecyl sulfosuccinate inmixture of ethanol and water. NaSul CA-HT3 is calcium dinonylnaphthalenesulfonate/carboxylate complex. C500 is an oil soluble calcium sulfonate.

For an anionic surfactant, the counter ion is typically sodium but mayalternatively be potassium, lithium, calcium, magnesium, ammonium,amines (primary, secondary, tertiary or quandary) or other organicbases. Exemplary amines include isopropylamine, ethanolamine,diethanolamine, and triethanolamine. Mixtures of the above cations mayalso be used.

The surfactants for use in the present invention may also be cationic,so long as at least one surfactant bearing a net positive charge is alsoincluded. Such cationic surfactants include, but are not limited to,primarily organic amines, primary, secondary, tertiary or quaternary.For a cationic surfactant, the counter ion can be chloride, bromide,methosulfate, ethosulfate, lactate, saccharinate, acetate and phosphate.Examples of cationic amines include polyethoxylated oleyl/stearyl amine,ethoxylated tallow amine, cocoalkylamine, oleylamine, and tallow alkylamine.

Examples of quaternary amines with a single long alkyl group are cetyltrimethyl ammonium bromide (CETAB), dodecyltrimethylammonium bromide,myristyl trimethyl ammonium bromide, stearyl dimethyl benzyl ammoniumchloride, oleyl dimethyl benzyl ammonium chloride, lauryl trimethylammonium methosulfate (also known as cocotrimonium methosulfate),cetyl-dimethyl hydroxyethyl ammonium dihydrogen phosphate,bassuamidopropylkonium chloride, cocotrimonium chloride,distearyldimonium chloride, wheat germ-amidopropalkonium chloride,stearyl octyidimonium methosulfate, isostearaminopropal-konium chloride,dihydroxypropyl PEG-5 linoleammonium chloride, PEG-2 stearmoniumchloride, behentrimonium chloride, dicetyl dimonium chloride, tallowtrimonium chloride and behenamidopropyl ethyl dimonium ethosulfate.

Examples of quaternary amines with two long alkyl groups aredistearyldimonium chloride, dicetyl dimonium chloride, stearyloctyldimonium methosulfate, dihydrogenated palmoylethylhydroxyethylmonium methosulfate, dipalmitoylethyl hydroxyethylmoniummethosulfate, dioleoylethyl hydroxyethylmonium methosulfate, andhydroxypropyl bisstearyldimonium chloride.

Quaternary ammonium compounds of imidazoline derivatives include, forexample, isostearyl benzylimidonium chloride, cocoyl benzyl hydroxyethylimidazolinium chloride, cocoyl hydroxyethylimidazolinium PG-chloridephosphate, and stearyl hydroxyethylimidonium chloride. Otherheterocyclic quaternary ammonium compounds, such as dodecylpyridiniumchloride, can also be used.

The surfactants for use in the present invention may be nonionic,including, but not limited to, polyalkylene oxide carboxylic acidesters, fatty acid esters, fatty alcohols, ethoxylated fatty alcohols,poloxamers, alkanolamides, alkoxylated alkanolamides, polyethyleneglycol monoalkyl ether, and alkyl polysaccharides. Polyalkylene oxidecarboxylic acid esters have one or two carboxylic ester moieties eachwith about 8 to 20 carbons and a polyalkylene oxide moiety containingabout 5 to 200 alkylene oxide units. A ethoxylated fatty alcoholcontains an ethylene oxide moiety containing about 5 to 150 ethyleneoxide units and a fatty alcohol moiety with about 6 to about 30 carbons.The fatty alcohol moiety can be cyclic, straight, or branched, andsaturated or unsaturated. Some examples of ethoxylated fatty alcoholsinclude ethylene glycol ethers of oleth alcohol, steareth alcohol,lauryl alcohol and isocetyl alcohol. Poloxamers are ethylene oxide andpropylene oxide block copolymers, having from about 15 to about 100moles of ethylene oxide. Alkyl polysaccharide (“APS”) surfactants (e.g.alkyl polyglycosides) contain a hydrophobic group with about 6 to about30 carbons and a polysaccharide (e.g., polyglycoside) as the hydrophilicgroup. An example of commercial nonionic surfactant is FOA-5 (OctelStarreon LLC., Littleton, Colo.).

Specific examples of suitable nonionic surfactants include alkanolamidessuch as cocamide diethanolamide (“DEA”), cocamide monoethanolamide(“MEA”), cocamide monoisopropanolamide (“MIPA”), PEG-5 cocamide MEA,lauramide DEA, and lauramide MEA; alkyl amine oxides such as lauramineoxide, cocamine oxide, cocamidopropylamine oxide, andlauramidopropylamine oxide; sorbitan laurate, sorbitan distearate, fattyacids or fatty acid esters such as lauric acid, isostearic acid, andPEG-150 distearate; fatty alcohols or ethoxylated fatty alcohols such aslauryl alcohol, alkylpolyglucosides such as decyl glucoside, laurylglucoside, and coco glucoside.

The surfactants for use in the present invention may be zwitterionic,which has both a formal positive and negative charge on the samemolecule. The positive charge group can be quaternary ammonium,phosphonium, or sulfonium, whereas the negative charge group can becarboxylate, sulfonate, sulfate, phosphate or phosphonate. Similar toother classes of surfactants, the hydrophobic moiety may contain one ormore long, straight, cyclic, or branched, aliphatic chains of about 8 to18 carbon atoms. Specific examples of zwitterionic surfactants includealkyl betaines such as cocodimethyl carboxymethyl betaine, lauryldimethyl carboxymethyl betaine, lauryl dimethyl alpha-carboxyethylbetaine, cetyl dimethyl carboxymethyl betaine, laurylbis-(2-hydroxyethyl)carboxy methyl betaine, stearylbis-(2-hydroxypropyl)carboxymethyl betaine, oleyl dimethylgamma-carboxypropyl betaine, and laurylbis-(2-hydroxypropyl)alphacarboxy-ethyl betaine, amidopropyl betaines;and alkyl sultaines such as cocodimethyl sulfopropyl betaine,stearyidimethyl sulfopropyl betaine, lauryl dimethyl sulfoethyl betaine,lauryl bis-(2-hydroxyethyl)sulfopropyl betaine, andalkylamidopropylhydroxy sultaines.

The surfactants for use in the present invention may be amphoteric.Examples of suitable amphoteric surfactants include ammonium orsubstituted ammonium salts of alkyl amphocarboxy glycinates and alkylamphocarboxypropionates, alkyl amphodipropionates, alkylamphodiacetates, alkyl amphoglycinates, and alkyl amphopropionates, aswell as alkyl iminopropionates, alkyl iminodipropionates, and alkylamphopropylsulfonates. Specific examples are cocoamphoacetate,cocoamphopropionate, cocoamphodiacetate, lauroamphoacetate,lauroamphodiacetate, lauroamphodipropionate, lauroamphodiacetate,cocoamphopropyl sulfonate, caproamphodiacetate, caproamphoacetate,caproamphodipropionate, and stearoamphoacetate.

The surfactants for use in the present invention may also be a polymersuch as N-substituted polyisobutenyl succinimides and succinates, alkylmethacrylate vinyl pyrrolidinone copolymers, alkylmethacrylate-dialkylaminoethyl methacrylate copolymers,alkylmethacrylate polyethylene glycol methacrylate copolymers, andpolystearamides.

Alternatively, the surfactant may be an oil-based dispersant, whichincludes alkylsuccinimide, succinate esters, high molecular weightamines, and Mannich base and phosphoric acid derivatives. Some specificexamples are polyisobutenyl succinimide-polyethylenepolyamine,polyisobutenyl succinic ester, polyisobutenylhydroxybenzyl-polyethylenepolyamine, and bis-hydroxypropyl phosphorate.

The surfactant used in the present invention may also be a combinationof two or more selected from the group consisting of anionic, cationic,nonionic, zwitterionic, amphoteric, and ampholytic surfactants. Suitableexamples of a combination of two or more surfactants of the same typeinclude, but are not limited to, a mixture of two anionic surfactants, amixture of three anionic surfactants, a mixture of four anionicsurfactants, a mixture of two cationic surfactants, a mixture of threecationic surfactants, a mixture of four cationic surfactants, a mixtureof two nonionic surfactants, a mixture of three nonionic surfactants, amixture of four nonionic surfactants, a mixture of two zwitterionicsurfactants, a mixture of three zwitterionic surfactants, a mixture offour zwitterionic surfactants, a mixture of two amphoteric surfactants,a mixture of three amphoteric surfactants, a mixture of four amphotericsurfactants, a mixture of two ampholytic surfactants, a mixture of threeampholytic surfactants, and a mixture of four ampholytic surfactants.

Suitable examples of a combination of two surfactants of different typesinclude, but are not limited to, a mixture of one anionic and onecationic surfactant, a mixture of one anionic and one nonionicsurfactant, a mixture of one anionic and one zwitterionic surfactant, amixture of one anionic and one amphoteric surfactant, a mixture of oneanionic and one ampholytic surfactant, a mixture of one cationic and onenonionic surfactant, a mixture of one cationic and one zwitterionicsurfactant, a mixture of one cationic and one amphoteric surfactant, amixture of one cationic and one ampholytic surfactant, a mixture of onenonionic and one zwitterionic surfactant, a mixture of one nonionic andone amphoteric surfactant, a mixture of one nonionic and one ampholyticsurfactant, a mixture of one zwitterionic and one amphoteric surfactant,a mixture of one zwitterionic and one ampholytic surfactant, and amixture of one amphoteric and one ampholytic surfactant. A combinationof two or more surfactants of the same type, e.g., a mixture of twoanionic surfactants, is also included in the present invention.

Other Chemical Additives:

The nanofluids of the present invention may also contain one or moreother chemicals to provide other desired chemical and physicalproperties and characteristics, depending on whether they arehydrophobic or hydrophilic. In addition to the chemicals discussedseparately below for hydrophilic and hydrophobic thermal transferfluids, many other known types of additives such as dyes and air releaseagents, can also be included in finished compositions produced and/orused in the practice of the present invention. In general, the additivecomponents are employed in nanofluids in minor amounts sufficient toenhance the performance characteristics and properties of the basefluid. The amounts will thus vary in accordance with such factors as theviscosity characteristics of the base fluid employed, the viscositycharacteristics desired in the finished fluid, the service conditionsfor which the finished fluid is intended, and the performancecharacteristics desired in the finished fluid.

Suitable additional chemical additives include, but are not limited to,buffering agents, corrosion inhibitors, defoamers, and scale inhibitors.

The buffering agents may be selected from any known or commonly usedbuffering agents. It will be appreciated by those skilled in the artthat selected buffering agents can exhibit both anti-corrosion andbuffering properties. In certain formulations, for example, benzoates,borates, and phosphates can provide both buffering and anti-corrosionadvantages. In addition a base can be used to adjust the pH value of ananofluid. Illustrative examples of bases for use with this inventioninclude commonly known and used bases, for example, inorganic bases suchas KOH, NaOH, NaHCO₃, K₂CO₃, and Na₂CO₃. Therefore, the buffering systemand base can be adapted to provide a nanofluid composition with a pHlevel between 7.5 and about 11.

The corrosion inhibitors may be either an organic additive or aninorganic additive. Suitable organic anti-corrosive additives includeshort aliphatic dicarboxylic acids such as maleic acid, succinic acid,and adipic acid; triazoles such as benzotriazole and tolytriazole;thiazoles suchs as mercaptobenzothiazole; thiadiazoles such as2-mercapto-5-hydrocarbylthio-1,3,4-thiadiazoles,2-mercapto-5-hydrocarbyldithio-1,3,4-thiadiazoles,2,5-bis(hydrocarbylthio)-1,3,4-thiadiazoles, and2,5-(bis)hydrocarbyldithio)-1,3,4-thiadiazoles; sulfonates; andimidazolines. Suitable inorganic additives include borates, phosphates,silicates, nitrates, nitrites, and molybdates.

Suitable defoamers include components such as silicon defoamers,alcohols such as polyethoxylated glycol, polypropoxylated glycol oracetylenic glycols.

Suitable scale inhibitors include components such as phosphate esters,phosphino carboxylate, polyacrylates, polymethacylate, styrene-maleicanhydride, sulfonates, maleic anhydride co-polymer, andacrylate-sulfonate co-polymer.

The basic composition of the nanofluids of the present invention can beroutinely optimized for selective applications. For example, nitratesand silicates are known to provide aluminum protection. Borates andnitrites can be added for ferrous metal protection, and benzotriazoleand tolytriazole can be added for copper and brass protection.

Physical Agitation:

The nanofluid of the present invention is prepared by dispersing amixture of the appropriate carbon nanoparticles, metal oxidenanoparticles and surfactant(s), and other optional chemical additives,using a physical method to form a stable suspension of nanoparticles ina thermal transfer fluid. A variety of physical mixing methods aresuitable for use in the present invention, including a conventionalmortar and pestle mixing, high shear mixing, such as with a high speedmixer, homogenizers, microfluidizers, high impact mixing, andultrasonication methods.

Among these methods, ultrasonication is the least destructive to thestructures of carbon nanoparticles. Ultrasonication can be done eitherin the bath-type ultrasonicator, or by the tip-type ultrasonicator.Typically, tip-type ultrasonication is for applications which requirehigher energy output. Ultrasonication at a intermediate intensity for upto 60 minutes, and usually in a range of from 10 to 30 minutes isdesired to achieve better homogeneity. Additionally, the mixture isultrasonicated intermittently to avoid overheating. It is well knownthat overheating can cause covalent bond breakage in a carbon nanotube,which cause the nanotube to lost its beneficial physical properties. Assuch, the carbon nanoparticle-containing mixture is generally energizedfor a predetermined period of time with a break in between. Eachenergizing period is no more than about 30 min, no more than about 15min, no more than 10 min, no more than 5 min, no more than 2 min, nomore than 1 min, or no more than 30 sec. The break betweenultrasonication pulses provides the opportunity for the energized carbonnanoparticles to dissipate the energy. The break is typically no lessthan about 1 min, no less than about 2 min, no less than about 5 min, orbetween about 5 to about 10 min.

The raw material mixture may also be pulverized by any suitable knowndry or wet grinding method. One grinding method includes pulverizing theraw material mixture in the fluid mixture of the present invention toobtain a concentrate, and the pulverized product may then be dispersedfurther in a liquid medium with the aid of the dispersants describedabove. However, pulverization or milling often reduces the carbonnanotube average aspect ratio.

It will be appreciated that the individual components can be separatelyblended into the thermal transfer fluid, or can be blended therein invarious subcombinations, if desired. Ordinarily, the particular sequenceof such blending steps is not critical. Moreover, such components can beblended in the form of separate solutions in a diluent. It ispreferable, however, to blend the components used in the form of anadditive concentrate, as this simplifies the blending operations,reduces the likelihood of blending errors, and takes advantage of thecompatibility and solubility characteristics afforded by the overallconcentrate.

Physical agitation methods particularly suitable for making nanogreaseare those employing relatively high shearing or dispersing devices,including, but not limited to, Morehouse mills, Buxton knife mills,Gaulin homogenizers, colloid mills, rotating knife-edge mills,rotor-stator mills, and three-roll mills. In an exemplary embodiment,after a final grease composition is achieved, the resulting grease isgenerally passed one or more times through one of these shearing ordispersing devices to enhance the characteristics (e.g., smoothness,shear stability, oil separation and bleed properties) and to maximizethe thickening power of a grease thickener, such as carbon nanotubes.

Formulation:

The nanofluid of the present invention is a dispersion of carbonnanoparticles, metal oxide nanoparticles and at least one surfactantwith a net negative charge in a thermal transfer fluid. In general, thenanofluid contains no less than about 80%, no less than about 85%, noless than about 90%, or no less than about 95% by weight of a thermaltransfer fluid. The nanofluid contains no greater than about 30%, nogreater than 15%, no greater than 10%, no greater than about 5%, nogreater than about 2.5%, or no greater than about 1%, no greater thanabout 0.5%, no greater than about 0.2%, no greater than about 0.1%, orno greater than about 0.05% by weight of carbon or non-carbonnanoparticles.

The nanofluid includes one or more surfactants with a net negativecharge to stabilize the nanoparticle dispersion. The nanofluid containsfrom about no greater than 10%, no greater than 1%, no greater than0.5%, no greater than 0.2%, from 0.1 to about 30%, from about 1 to about20%, from about 1 to about 15%, or from about 1 to about 10% by weightof a surfactant.

The nanofluid may further comprise other additives to improve chemicaland/or physical properties. Typically, the amount of these additivestogether is no greater than 10% by weight of the nanofluid.Nevertheless, the total amount of all the ingredients of the nanofluidtogether should equal to 100%.

The metal oxide nanoparticles of the present invention exhibit apositive surface charge. In addition, such metal oxides all have acharacteristic pH point of zero charge, or “pHpzc” at which pH the metaloxide nanoparticle surface is neutral. In the practice of the presentinvention, the pH of the nanofluid is adjusted to below this pHpzc. Thisforms an additional positively charged ionic barrier around the metaloxide nanoparticles that facilitates inter-particle repulsion betweenparticles, which further enhances the stability of the dispersion. Inone example, when the metal oxide nanoparticles have a pHpzc of between9 and 10, the pH of the nanofluid is usually between 7 and 9. In aspecific example, the nanofluids comprise SWNTs, along with metal oxide(MgO, CuO or Al₂O₃) nanoparticles and SDS or SDBS at a pH of between 7and 9.

By providing a transfer fluid having an appropriate pH, a charge effectbetween the surfactant molecules and the non-carbon nanoparticles can bemaintained. Carbon nanoparticles can then be maintained in suspensiondue to the charge effect between the head groups on the surfactantmolecules.

As mentioned above, the net charge of the surfactant(s) is(are)negative, and the metal oxide nanoparticles have a positive charge. Thisallows a small network of metal oxide nanoparticles to form, therebyorienting and connecting the carbon nanoparticle, which in turn allowsheat to flow along the length of the carbon nanoparticle such as acarbon nanotube, thereby significantly increasing the thermalconductivity of the nanofluid.

Such nanofluids can be exhibited microscopically to form large ropestructure comprising many carbon nanotubes that have been partiallyexfoliated into smaller ropes through the use of the surfactants, suchas NaDDBS. In other examples individual carbon nano-tubes that areobtained by the addition of MgO at pH 7 with the surfactant NaDDBS maybe coated with what is believed to be a random assortment of surfactanton the surface of the carbon nano-tubes. These nanofluids can exyibitring formations.

The hydrophilic thermal transfer fluid may contain one or morehydrophilic molecules. For example, the hydrophilic thermal transferfluid may contain water, aliphatic alcohols, alkylene glycols, orvarious mixtures thereof. The hydrophilic thermal transfer fluid may bea two-component mixture which contains water and ethylene glycol invarious proportions. The hydrophilic thermal transfer fluid may containabout 0.1 to about 99.9%, about 20 to about 80%, about 40 to about 60%,or about 50% by volume of water.

The hydrophilic thermal transfer fluid may be a three-component mixture.For example, the hydrophilic thermal transfer fluid contains water,ethylene glycol, and diethylene glycol in various proportions. Thehydrophilic thermal transfer fluid may contain about 0.1 to about 99.9%by volume of water, about 0.1 to 99.9% by volume of ethylene glycol, andabout 0.1 to 99.9% by volume of diethylene glycol; and about 20 to about80%, about 40 to about 60%, or about 50% by volume of water or ethyleneglycol. Typically, diethylene glycol constitutes a minor component ofthe hydrophilic thermal transfer fluid, in no greater than about 20%, nogreater than about 10%, or no greater than about 5% of the total volume.Nevertheless, the total amount of all the components in a hydrophilicthermal transfer fluid together equals to 100%.

The hydrophilic nanofluid may be prepared by dispersing carbonnanoparticles and metal oxide nanoparticles, along with thesurfactant(s) directly into a mixture of a hydrophilic thermal transferfluid and other additives with a physical agitation, such asultrasonication. However, the order of addition of the individualcomponents is not critical for the practice of the invention.

The hydrophilic nanofluid of carbon nanoparticles thus produced hasenhanced thermal properties and physical and chemical characteristics.Addition of solid carbon nanoparticles, in particular, carbon nanotubes,enhances both thermal conductivity and lowers the freezing point of thethermal conductive fluid. Incorporation of about 0.05% by weight ofcarbon nanotubes, the thermal conductivity is increased from 0.45 toabout 0.48-0.50, which is an about 6 to 11% increase. In addition, thefreezing point of the thermal transfer fluid is also loweredsignificantly.

EXAMPLES

Carbon nanotubes from several commercial sources were used in thefollowing examples and their information is summarized in Table 1.Specifically, SWNT-CNI and SWNT-RIC were produced using a chemical vapordeposition process (“VCD”), whereas SWNT-CAR was produced via an arcdischarge method.

Often, several grades of carbon nanotubes are available from eachcompany. For instance, CNI supplies “D” grade, HiPCO purified, andfluorinated carbon nanotubes. The “D” grade carbon nanotubes containroughly 35% ash content along with some metal catalysts impurities.HiPCO purified and fluorinated carbon nanotubes contain less than 10%ash. Fluorinated carbon nanotubes have their end cap functionalized withfluorine. The carbon nanotubes from Carbolex (SWNT-CAR) are AP gradewithout purification. The carbon nanotubes from Rice University havebeen functionalized with sulfonated aryl groups, allowing them to bemore easily dispersed in polar solvents such as water and methanol.

Metal oxide nanoparticles used in the present invention are magnesiumoxide (MgO) with an average diameter of 12 nm, aluminum oxide (Al₂O₃)with an average diameter of 40 nm, copper oxide (CuO) with an averagediameter of 33 nm, and magnesium hydroxide (Mg(OH)₂) with an averagediameter of 26.8 nm. These metal oxide nanoparticles were all obtainedfrom Sigma Aldrich (St. Louis, Mo.).

In the following examples, thermal conductivity was measured using a HotDisk™ Thermal Constant Analyzer (Uppsala, Sweden). The senor was a 2 mmnickel double spiral sensor covered by a thin kapton layer. After eachmeasurement, the sample was allowed to sit for 15 min before the nextmeasurement. Both particle size and zeta-potential were determined usingNICOMP 380 ZLS by Particle Sizing Systems. Zeta potential was measuredby using the electrophoretic light scattering method (ELS). Particlesize is obtained by use of the multi-angle dynamic light scatteringtechnique (DLS). Ultrasonication was performed using a Branson model 450ultrasonicator with a half-inch disruptor horn. Transmission electronmicroscopy (TEM) was performed using the Hitachi H-7000FA ElectronMicroscope.

TABLE 1 Carbon Nanotubes Commercial Abbreviation Product InformationSource MWNT-HMSI MWNT with a diameter of 10-20 Helix Material nm and alength of 0.5-40 Solution Inc micrometers MWNT-MER MWNT with a diameterof Materials and 140 ± 30 nm, a length of 7 ± 2 Electrochemicalmicrometers, and a purity of Research over 90%. Corporation MWNT-RAOMWNT with diameter 20-25 nm, RAO SWNT-MER SWNT 0.7-1.2 nm in diameter,10-50 MER micron lengths. SWNT-CAR Purified CAR SWNT (AP CAR) CarboLexSWNT-D-CNI Grade-D SWNT CNI F-SWNT-CNI Purified F-SWNT CNI SWNT-H-CNIHiPco SWNT CNI SWNT-RIC Functionalized SWNT Rice University

Example 1 Thermal Conductivities of Hydrophilic Thermal Transfer Fluids

For comparison with carbon nanoparticle-containing nanofluids, thethermal conductivities of hydrophilic thermal fluids used in the presentinvention were measured. The thermal conductivity of distilled water wasdetermined to be 0.628±0.003 W/mK, which is slightly higher than thereported value of 0.613 W/mK (Eastman, et al., “Thermal transport innanofluids,” Annual Review of Materials Research, 2004, 34:219-246). Thethermal conductivity of ethylene glycol was determined to be0.2731±0.003 W/mK, which is also slightly higher than the value of 0.256W/mK as reported in the aforementioned reference. The thermalconductivity of a solution containing 50% by volume of ethylene glycoland 50% by volume of water was determined to be 0.415±0.006 W/mK. Thethermal conductivity of PRESTONE™, which contains 50% by volume ofethylene glycol and 50% by volume of water, was determined to be0.434±0.002 W/mK. The thermal conductivities of these two ethyleneglycol solutions are very similar to the value of 0.434 W/mK as reportedin the aforementioned reference.

Example 2 Thermal Conductivities of Hydrophilic Nanfluids ComprisingSWNT and MgO

A series of hydrophilic nanofluids that contains SWNT, MgOnanoparticles, and SDBS in deionized water were prepared using a mortarand pestle via hand. The SWNTs used herein are SWNT-D-CNI (Grade D). Thefinal pH of the dispersions was 7. The resulting nanofluids were stablefor at least three months. The thermal conductivities of thesenanofluids are summarized in Table 2, together with their compositions.Surprisingly, the SWNT concentrations in the nanofluids do not seem tohave significant effects on their thermal conductivities.

TABLE 2 Thermal Conductivities of Hydrophilic Nanofluids Containing SWNTand MgO Thermal SWNT MgO SDBS Conductivity Average (wt. %) (wt. %) (wt.%) (W/mK) (W/mK) 0.017 0.05  0.17 0.6781 0.6650 ± 0.015 0.6677 0.64930.033 0.033 0.33 0.6618 0.6627 ± 0.005 0.6600 0.6559 0.6684 0.6673 0.0330.017 0.17 0.6755  0.6643 ± 0.0073 0.6641 0.6663 0.6584 0.6574

Example 3 Effect of pH on the Thermal Conductivities of HydrophilicNanfluids Comprising SWNT and MgO

It was observed that the pH value of the nanofluid plays an importantrole in its thermal conductivity and stability. The nanofluid that hasSWNT and MgO is stable at pH below 8 and its thermal conductivity alsodoes vary much. However, when the pH of the nanofluid was adjustedaround 10 or higher, the particles in the nanofluid started to separatedfrom the suspension and led to a decrease in its thermal conductivity(Table 3). The hydrophilic nanofluid in this study comprises 0.017% byweight of SWNT-D-CNI, 0.017% by weight of MgO, and 0.17 by weight ofSDBS at pH 10 in water.

TABLE 3 Thermal Conductivity of A Hydrophilic Nanofluid Containing SWNTand MgO at pH 10 Time (min) Thermal conductivity (W/mK) 15 0.6709 ±0.005 30 0.6452 ± 0.006 45 0.6294 ± 0.005

Example 4 Effect of the Grade of SWNT on the Thermal Conductivity of aHydrophilic Nanfluid Comprising SWNT and MgO

Three different grades of SWNTs were used in this study and the resultsare summarized in Table 4. Each hydrophilic nanofluid in the studycomprises 0.017% by weight of SWNT, 0.017% by weight of MgO, and 0.17 byweight of SDBS at pH 7 in deionized water. No significant effect of theSWNT grade was observed on the thermal conductivities of the hydrophilicnanofluids.

TABLE 4 The Effect of the Grade of SWNT on the Thermal ConductivityThermal Conductivity Average SWNT (W/mK) (W/mK) SWNT-D-CNI 0.6755 0.6643± 0.0073 0.6641 0.6663 0.6584 0.6574 SWNT-H-CNI 0.6781 0.6699 ± 0.01160.6617 SWNT-AP-CAR 0.6470 0.6476 ± 0.002  0.6499 0.6460

Example 5 Effect of Surfactants on Hydrophilic Nanfluids Comprising SWNTand MgO

Three different surfactants were tested, SDBS, SDS, and CTAB. The twoanionic surfactants, SDBS and SDS, work equivalently well in dispersingthe carbon nanotubes in water, and water and ethylene glycol mixtures.However, the nanofluid that contains the cationic CTAB exhibits a poorstability.

When dispersed via ultrasonication for 90 seconds, the resulted anionicsurfactant-containing nanofluid was stable for a few days. TheSDS-containing nanofluid shows a thermal conductivity of 0.6565±0.007W/mK, which is similar to that of the SDBS-containing nanofluid asdescribed herein above. For anionic surfactants, the optimal ratio ofsurfactant vs. the carbon nanotubes for effective dispersion of carbonnanotubes was found to be about 10:1 (surfactant:carbon nanotubes, w/w).

Example 6 Effect of Non-Carbon Nanoparticles on SWNT-ContainingHydrophilic Nanfluids

In addition to MgO, two additional non-carbon nanoparticles were used inthis study, CuO and Al₂O₃. CuO has a pH point of zero charge (“pHpzc”)of 9.5, whereas Al₂O₃ has a pHpzc of 9.2. The pH effect on the thermalconductivities of the Al₂O₃-containing nanofluids suggests that the pHshould be no greater than the pHpzc value of the non-carbonnanoparticle. Each nanofluid in Table 5 comprises 0.017% by weight ofSWNT, 0.017% by weight of non-carbon nanoparticlues, and 0.17 by weightof SDBS at the pH as indicated in water.

TABLE 5 The Effect of Non-carbon Nanoparticles on the ThermalConductivities of Hydrophilic Nanofluids Non-Carbon Thermal ConductivityAverage Nanoparticles pH (W/mK) (W/mK) CuO 7 0.6839 0.6816 ± 0.0040.6842 0.6768 Al₂O₃ 7 0.6541 0.6658 ± 0.008 0.6716 0.6724 0.6654 Al₂O₃ 50.6743 0.6696 ± 0.008 0.6632 0.6669 0.6818 0.6619 Al₂O₃ 10.75 0.62520.6269 ± 0.007 0.6212 0.6344

Example 7 Thermal Conductivities of Hydrophilic Nanofluids ContainingFunctionalized SWNT

For comparison, a series of hydrophilic nanofluids that containfunctionalized SWNT-RIC were also prepared. Due to the polar sulfonatedaryl group attached to the sidewall of the SWNT, SWNT-RIC was readilydispersed into water, a 50-50 water and ethylene glycol mixture, andethylene glycol. The dispersions were stable over long time periods.Dispersions of the SWNT-RIC were obtained through stirring with a glassrod, or via a few second ultrasonication (10 seconds or less).

The thermal conductivity of a nanofluid that comprises 0.5% by weight ofSWNT-RIC in water was determined to be 0.6397±0.0006 W/mK. The thermalconductivity of a nanofluid that comprises 0.1% by weight of SWNT-RIC inwater was determined to be 0.6389±0.0005 W/mK.

The thermal conductivity of a nanofluid that comprises 0.5% by weight ofSWNT-RIC in a solution that contains 50% by volume of water and 50% byvolume of ethylene glycol was determined to be 0.4441±0.0190 W/mK. Thethermal conductivity of a nanofluid that comprises 0.1% by weight ofSWNT-RIC in a solution that contains 50% by volume of water and 50% byvolume of ethylene glycol was determined to be 0.4483±0.0228 W/mK. Thereseems to be a slight increase in thermal conductivity from 0.415 W/mK toabout 0.444 W/mK. It is interesting to note that the solution is stableat a pH of 10, but if the pH is changed to 7, the solution of SWNT-RICstarted to agglomerate and the dispersion worsens considerably, leadingto decrease in thermal conductivity.

The examples set forth above are provided to give those of ordinaryskill in the art with a complete disclosure and description of how tomake and use the preferred embodiments of the compositions and themethods, and are not intended to limit the scope of what the inventorsregard as their invention. Modifications of the above-described modesfor carrying out the invention that are obvious to persons of skill inthe art are intended to be within the scope of the following claims. Allpublications, patents, and patent applications cited in thisspecification are incorporated herein by reference as if each suchpublication, patent or patent application were specifically andindividually indicated to be incorporated herein by reference.

1. A nanofluid composition comprising carbon nanoparticles, metal oxidenanoparticles having a pHpzc, and at least one surfactant having a netnegative charge in a hydrophilic thermal transfer fluid, wherein thecomposition has a pH below the pHpzc of the metal oxide nanoparticles.2. The nanofluid composition of claim 1, wherein the carbonnanoparticles are selected from the group consisting of diamondnanoparticles, graphite nanoparticles, fullerenes, carbon nanotubes, andcarbon fibers.
 3. The nanofluid composition of claim 1, wherein thehydrophilic thermal transfer fluid is selected from the group consistingof water, alkyl alcohols, alkylene glycols, and combinations thereof. 4.The nanofluid composition of claim 3, wherein the alkylene glycol isethylene glycol or diethylene glycol.
 5. The nanofluid composition ofclaim 1, wherein the carbon nanoparticles are carbon nanotubes.
 6. Thenanofluid composition of claim 1, wherein the carbon nanoparticles aresingle wall nanotubes (SWNTs).
 7. The nanofluid composition of claim 1,wherein the carbon nanoparticles are multi-wall nanotubes (MWNTs). 8.The nanofluid composition of claim 1, wherein the metal oxidenanoparticle comprises a metal oxide selected from the group consistingof MgO, CuO and Al₂O₃.
 9. The nanofluid composition of claim 1, whereinthe surfactant is selected from the group consisting of sodiumdodecylbenzenesulfonate (SDBS) and sodium dodecyl sulfate (SDS).
 10. Thenanofluid composition of claim 1, wherein the carbon nanotube is a SWNT,the metal oxide nanoparticle comprises MgO, CuO or Al₂O₃, at least onesurfactant is SDS or SDBS, and the composition has a pH of between 7 and9.
 11. A method for making a nanofluid composition comprising the stepsof: a) mixing together carbon nanoparticles, metal oxide nanoparticleshaving a pHpzc, and at least one surfactant having a net negative chargeinto a hydrophilic thermal transfer fluid; and b) adjusting the pH ofthe composition below the pHpzc of the metal oxide nanoparticles.